This invention relates to devices of the type in which liquid samples are analyzed by causing infrared radiation (IR) inside a transparent optical element to be partially absorbed by a liquid sample in which the element is immersed. The optical elements, formed of such materials as zinc selenide, germanium, etc., are often referred to as internal reflectance elements, and the units inside which they interface with the sample are often referred to as attenuated total reflectance (ATR) sample cells.
The present invention is primarily concerned with ATR cells in which the internal reflecting element (IRE) is circular in cross-section. The optical aspects and advantages of such circular internal reflectance (CIR) devices are discussed in detail in U.S. application Ser. No. 312,130, which was filed Feb. 17, 1989, and which has the same assignee as the present application.
Experience with CIR cells has uncovered certain problems of a mechanical, rather than optical, nature.
Although CIR cells are widely used to analyze both stationary and flowing liquids, a number of problems have been encountered with the flow jacket, or housing, designs being used. For one thing, the liquid flow velocity varies significantly from region to region in the sample cell. In fact, there are often regions referred to as "dead volume", in which the liquid flow rate is low and, as a result, sample material tends to accumulate. Often this material is not fully removed when the cell is flushed out with a solvent. This leads to spurious bands in the spectra of subsequent samples.
A second problem occurs when a cell is being used for the analysis of continuously flowing substances, but where either the flow rate is low or the sample is quite viscous. In such situations, the progressive build up of sample material in the cell can lead not only to an improper spectral time history of the process, but also in some cases to clogging of the cell.
A third problem occur when the flowing liquid to be analyzed contains catalysts or other abrasives. These can gradually abrade the relatively soft internal reflectance element (IRE), leading to a progressive decrease in IR transmission. This problem is exacerbated by the fact that the liquid input to the cell in prior designs is directed against the surface of the IRE.